1. Field of the Invention
The present invention relates to a gas detection device using a semiconductor laser, and, in particular, to a gas detection device suitable for remote quantitative gas concentration measurements.
2. Description of the Prior Art
Methods of detecting the presence of a gas, utilizing the characteristics of gases to absorb laser beams of specific wavelengths, are commonly known. Sensing technology based on this principle is widely used in industrial measurements, pollution monitoring, and the like. For example, methane strongly absorbs the 3,392 .mu.m emission of the He-Ne laser, but absorbs only slightly the 3,391 .mu.m emission of the same laser. Accordingly, it is possible to detect the presence of methane with good sensitivity from the difference between the strengths of the transmitted light when laser beams of two wavelengths with equal intensity strike the gas being probed. Because methane is the main constituent of city gas, it is possible to detect leaks of city gas by detecting the methane gas.
A methane leak sensor of this type is disclosed in U.S. Pat. No. 4,489,239. However, this sensor requires two lasers, two choppers, and two lock-in amplifiers, which makes the device voluminous. At the same time, this U.S. patent also proposes a laser device which oscillates at two wavelengths simultaneously and thus eliminates this drawback. However, some problems still remain, such as the difficulty in maintaining equal output powers at the two wavelengths.
Another known method uses one mechanical chopper to equalize the output powers at the two wave lengths from two separate lasers. However, a methane detection system of this type needs many mirrors and beam splitters, making the optical system very complicated. Therefore, optical alignment is troublesome and the laser power loss is large. Furthermore, the signal processing is complicated, and high frequency modulation to attain high signal-to-noise ratio (S/N) cannot be accomplished because the mechanical chopper is used.
Accordingly, the inventors of the present invention, in Japanese Laid Open Patent Application 62-290190, proposed a novel two-frequency oscillation gas laser device which eliminates the drawbacks of the laser device used in the above-mentioned conventional gas detection systems. This two-frequency gas laser device oscillates alternately at two different wavelengths. Feedback is provided to adjust the cavity length so that the intensities of the two components are equal. Gas can be detected with this concise structure in which only one laser oscillator is used.
However, this two-frequency gas laser device has some drawbacks, too. Firstly, the instability of the laser cavity due to thermal expansion and mechanical vibrations disturbs the feedback control of the laser. Therefore, a high sensitivity gas detection is difficult to achieve. This drawback becomes a more severe problem when a portable device used in the field is considered. Secondly, a rather high electric power is necessary to produce a high voltage to operate the gas laser device. This is also a problem for a portable device.